Seismic Site Classification Research Articles

Guidelines and pitfalls of refraction microtremor surveys

The geotechnical industry has widely adopted the refraction microtremor shear-wave velocity measurement technique, which is accepted by building authorities for evaluation of seismic site class around the world. Clark County and the City of Henderson, Nevada, populated their Earthquake Parcel Map with over 10,000 site measurements for building code enforcement, made over a 3-year period. 2D refraction microtremor analysis now allows engineers to image lateral shear-wave velocity variations and do passive subsurface imaging. Along with experience at a basic level, the ability to identify the “no energy area” and the “minimum-velocity envelope” on the slowness-frequency (p-f) image helps practitioners to assess the quality of their ReMi data and analysis. Guides for grading (p-f) image quality, and for estimating depth sensitivity, velocity-depth tradeoffs, and depth and velocity resolution also assist practitioners in deciding whether their refraction microtremor data will meet their investigation objectives. Commercial refraction microtremor surveys use linear arrays, and a new criterion of 2.2% minimum microtremor energy in the array direction allows users to assess the likelihood of correct results. Unfortunately, any useful and popular measurement technique can be abused. Practitioners must follow correct data collection, analysis, interpretation, and measurement procedures, or the results cannot be labeled “refraction microtremor” or “ReMi” results. We present some of the common mistakes and provide solutions with the objective of establishing a “best practices” template for getting consistent, reliable models from refraction microtremor measurements.

Site characterization of strong-motion stations of Himalaya and adjoining plains

Site investigation is crucial for any strong-motion station’s installation due to the modifications caused by underlying soil strata on strong-motion records. Hence, this study focuses on the geophysical exploration of 54 strong-motion stations located in the Himalaya and adjoining plains to estimate the shear wave velocity (Vs) profile and depth of bedrock of underlying soil strata. The methodology adopted in this study for site characterization is the joint inversion of the surface wave dispersion curve and horizontal to the vertical spectral ratio of microtremor. Joint inversion of these two methods helps reduce the uncertainty in the results and provides deep VS profiles up to the bedrock. Such VS profiles would improve the results of ground response analysis required for seismic hazard assessment. The study results are presented in the form of complete VS profiles and essential parameters such as VS30 and depth of engineering bedrock. Seismic site classification is also presented using VS30 as a proxy. Site-class evaluated in the present study for several sites are compared with the results of earlier studies. The comparison of the site class shows dissimilarities for few site locations. The dissimilarity could be due to the adoption of indirect methods for site classification by earlier studies. The sensitivity analysis is also performed for evaluating the effect of possible uncertainties in VS profiles on ground response analysis. The results suggest that the VS profiles and site classes estimated in this study shall be preferred for future seismic hazard studies of the region.

A Comparative Study of Response and Fourier Spectral Ratios on Classifying Sites

It has been known for decades that predominant periods of soil columns have crucial roles in the evaluation of site effects of ground motion. For this reason, detecting reliable site predominant periods relies upon a diligent data process and frequency domain transform method. In engineering seismology literature, predominant period-based site classifications have been developed to mitigate the local site effects of earthquakes. The main purpose of this study is to investigate the differences between Fourier and response spectral ratios in order to estimate the clear predominant periods and seismic site classes using instrumental accelerogram data. Within this framework, Fourier and response spectra of 26 installed strong ground motion stations in southern Turkey operated by AFAD [T.C. Disaster and Emergency Management Presidency, Strong Ground Motion Database of Turkey (TR-NSMN)] were examined. A total of 527 acceleration records with magnitude ranging from 3 to 5 having shallow crustal depths (0–25 km) and near-field earthquakes (< 150 km) have been analyzed. Elastic response and Fourier spectral ratios of horizontal-to-vertical (H/V) spectra were evaluated for a fair comparison using the S-wave phase. Although there are no significant differences, it was observed in this study that the response spectral ratio’s predominant periods reveal more reliable results for engineering purposes, especially in many sites where Fourier spectra show superficial peaks at engineering bedrock. As a pioneer study for a part of Turkey’s seismic station network, it will be an important step to establish a relationship between response and Fourier spectra to detect the site classes of stations alternatively in a global extent manner in the future.

Seismic Site Classification from Surface Wave Data to Vs,30 without Inversion

Surface wave methods have been extensively employed to estimate time-averaged shear-wave velocity (Vs) to 30 m (Vs,30) for seismic site classification. However, the inversion of surface wave dispersion data for Vs profiling is inherently computationally intensive. To overcome the challenge involved in surface-wave inversion, a new method is proposed to directly estimate Vs,30 from surface wave fundamental-mode dispersion data without inversion. The new method is based on the concept that the phase velocity of surface waves (VR) at a given frequency (f) is proportional to the average shear-wave velocity of soils within one-half of the wavelength (λ=VR/f). The proposed method starts from calculating the Vs of the top layer as it can be directly estimated from the phase velocity of surface waves whose half wavelengths are smaller than the thickness of the top layer. Likewise, the Vs of the second layer can be deduced from the top layer Vs along with the phase velocity of surface waves whose half wavelengths are within the thickness range of the second layer. Through such a repetition of sequence-deduction, the Vss of the other layers can be calculated. This method is verified with five case studies that cover typical geotechnical sites. Results indicate that the proposed method can obtain a similar Vs,30 with the same site classification for seismic design in comparison with invasive tests and inversion techniques.

Evaluation of seismic site classification for Kahramanmaras City, Turkey

This paper presents a study on the seismic site classification map using the geophysical tests in Kahramanmaras city located at a place where African, Anatolian, and Arabian plates meeting in southern-central Turkey. Generating seismic site classification maps in accordance with National Earthquake Hazards Reduction Program (NEHRP) has become a more significant criterion for earthquake hazard estimations. The SPT-N values obtained from the field studies at 287 boreholes within the upper 30 m were used to describe the subsurface conditions in the region. The shear wave velocity (VS) values in the study area were obtained by implementing Multichannel Analysis of Surface Waves (MASW) and Microtremor Array Method (MAM) measurements tests. An approach proposed by Boore (Boore, Bull Seismol Soc Am 94:591–597, 2004) for the cases where the VS measurements do not reach 30 m depth has also been adopted by correlating the shallow shear wave velocity with VS30. The resulting site classification maps estimate that the study area is predominantly classified as soil site class C, while the small areas were rarely classified as soil site class D and B. Furthermore, a systematic analysis based on a comparative study of the present research and the published correlations for seismic site classification with VS30 values has been carried out using Geographical Information System (GIS). Evidently, the VS30 based seismic site classification maps could be effectively used by researchers and engineers for the purpose of land-use planning and urban development in earthquake-prone regions.

System reliability analysis for seismic site classification based on sequential Gaussian co-simulation: A case study in Shiraz, Iran

Seismic site classification plays a significant role in the estimation of site response and is carried out using average shear wave velocity through the top 30 m (VS30) soil layers. Meanwhile, at some sites with a number of boreholes, identification of the VS30 acquired from different boreholes does not lead to the classification of the site into one single category, and this makes the seismic classification of the site troublesome. This paper proposes a practical approach for obtaining the reliability index of the seismic site classification when not all VS30 calculated from the boreholes converge to that of a unique seismic class. For this purpose, the geostatistical method is combined with the system reliability approach. A real site with three boreholes, two of which fall into the ‘D' and the third into the ‘C' category based on the National Earthquake Hazards Reduction Program (NEHRP), is considered. The site is divided into 66 hypothetical soil columns. The shear wave velocity (VS) is estimated in depths of each column using the Sequential Gaussian Co-Simulation (SGCS) method based on sampled data determined nearby. Ten thousand realizations of VS with spatial distributions are generated via the SGCS method and are validated with the original data. In order for reliability analysis of the proposed classification approach, the reliability indices of the components are merged based on their corresponding correlation through the Sequential Compounding Method (SCM). The proposed method is proven to be consistent, fast, and the results are shown to be reliable.

Design Response Spectra and Site Coefficients for Various Seismic Site Classes of Guwahati, India, Based on Extensive Ground Response Analyses

Seismicity of northeast India and its surroundings is governed by complex tectonic setting. Guwahati, the largest city of northeast India, had suffered minor to major damages during past earthquakes (EQs). Known seismic source information also suggests that Guwahati is susceptible for future EQs both from nearby as well as distance source. Taking into account the subsoil information of Guwahati, present work attempts to understand the role of Guwahati soil during probable seismic excitation. In the absence of regional ground motion records, covering a possible variation in ground motion characteristics, which can occur during potential future EQ, 30 globally recorded ground motions are used as input motions in this work. Each input motion is applied to each of the 225 boreholes resulting in 225 × 30 = 6750 ground response analyses using equivalent linear approach. Based on the response of subsoil to each input motion, site coefficients as well as average response spectra for various seismic site classes are proposed. It has to be mentioned here that in comparison to previous works which attempted to understand the subsoil of Guwahati based on specific location-based subsoil information and specific scenario EQ, present work provides a broader picture of soil response which can be applied to any level of ground shaking. Further, based on predominant period, suggestion of maximum building storeys are suggested for Guwahati. In addition, based on the work, characteristics of EQ scenario, which can trigger maximum response from local soil of Guwahati, are assessed. Based on the present analyses, design response spectra of Guwahati are also proposed. Keeping in mind Government of India’s policy towards the development of northeast India and further to develop Guwahati as “Smart city”, the present findings will be very useful in city planning, estimation of induced effects as well as for seismic design of probable infrastructure in Guwahati.

Site Amplification Factors and Acceleration Response Spectra for Shallow Bedrock Sites – Application to Southern India

ABSTRACT Site amplification coefficients and acceleration design response spectra (ADRS) for the shallow sites in SI are derived based on non-linear site response analysis at 125 locations. Input motions are selected by considering the seismic hazard map for 475 years as return period and inputted at depth where VS ≥ 1,500 m/s. The new site factors, i.e. , , and , are proposed for Southern India (SI) by classifying the sites seismically considering National Earthquake Hazards Reduction Program (NEHRP). Using the calculated site factor, a new ADRS is developed for SI, which depends on peak ground acceleration and seismic site class. The compatibility of ADRS is further evaluated with NEHRP and BIS:1893.

Influence of soil aging on SPT-Vs correlation and seismic site classification

This study examines the influence of soil aging on Standard Penetration Test (SPT) N-Vs correlations, and seismic site classification. Energy-corrected SPT-Vs correlations are developed for two regions where subsurface layering consists of soil deposits with very young geotechnical ages according to the last critical soil disturbance that occurred in the study areas. It is observed that the uncorrected correlations of the current study fall into the lower bound of the uncorrected SPT-Vs correlations existing in the literature due to soil aging effects. To better illustrate the effects of soil aging on SPT-Vs correlations, the uncorrected correlations from the literature and this study are divided into two age categories of Holocene and Pleistocene according to their geological age. Using the two age categories, the boundaries proposed in the NEHRP for different seismic site classes are divided into three zones according to their uncertainty level regarding seismic site classification. The results clearly indicate the need to consider the age of soil deposits for seismic site classification when using the SPT method to avoid incorrect seismic site classification. Additionally, it is found that the use of raw (uncorrected) SPT N values can lead to incorrect seismic site classification, with either conservative or unconservative results depending on the Energy Transfer Ratio (ETR) of the SPT hammer used for the seismic site class determination.

Influences of variations in the sledgehammer trajectory and collinearity of the geophone array in an MASW survey on the shear-wave velocity profile

In professional activities aimed at seismic site classification, geophysical methods based on measurements of surface waves are often used to measure the shear-wave velocity (Vs), with the MASW survey being one of the most common techniques employed for this purpose. An MASW survey is characterized by an in-field configuration that requires a relatively simple setup; however, several uncertainties that arise are related to the survey execution process. Thus, surface irregularities and/or obstacles on the ground surface in conjunction with possible human-related errors can result in alterations to the MASW survey execution. Therefore, it is necessary to possess a clear understanding of the variables that can potentially produce alterations in Vs profiles. The purpose of this study was to evaluate, based on a field measurement campaign, the effect of the repeatability of the sledgehammer trajectory prior to striking the plate and the effects of variations in the collinearity of the geophone arrays on the Vs profile in consideration of various forced alterations commonly encountered in practice. The repeatability of the active source trajectory plays a significant role in the quality and reliability of Vs measurements. Likewise, altering the collinearity of the geophone array leads to reductions in the amplitude at low frequencies, thereby hindering the interpretation of the test. The recorded effects become more relevant depending on the local conditions and whether the surveyed terrain exhibits possible heterogeneity.

UPDATING OF SURAKARTA SEISMIC SITE CLASS MAP BASED ON STANDARD PENETRTION TEST

&lt;p&gt;Surakarta is one of the cities in Java island with a high population growth rate, triggering the increase&lt;br /&gt;in the construction, especially for high-rise buildings. The city is close to several earthquake sources,&lt;br /&gt;causing a high level of seismic hazard. One of the main parameters of earthquake resistant building&lt;br /&gt;design is the value of peak ground acceleration at surface level. It is obtained by multiplying the peak&lt;br /&gt;ground acceleration in the bedrock by a multiplier factor that depends on the site class conditions of the&lt;br /&gt;location. A previous study has been conducted in 2015 using 32 boreholes indicating that around 19.5%&lt;br /&gt;of Surakarta area is classified as SC (hard soil), and 80.5% of the rest is SD (medium soil). To increase&lt;br /&gt;the accuracy, another 31 additional boreholes data are adopted. The new result shows that Surakarta is&lt;br /&gt;divided into three different seismic site class. Around 4.80% the area is classified as SC (hard soil),&lt;br /&gt;94.50% is SD (medium soil), and 0,70% is SE (soft soil). The southern part of Surakarta has a higher&lt;br /&gt;level of vulnerability to earthquakes, especially in the border area between Banjarsari, Laweyan, and&lt;br /&gt;Serengan subdistricts which have a SE (soft soil) seismic site class.&lt;/p&gt;

A new method of seismic site classification using HVSR curves: A case study of the 12 November 2017 Mw 7.3 Ezgeleh earthquake in Iran

On the 12th of November 2017, a strong earthquake with a moment magnitude of 7.3 struck the west of Iran, causing massive damage in SarpoleZahab and surrounding areas. In this study, the Horizontal to Vertical Spectral Ratio (HVSR or H/V technique) of strong ground motion recorded at 60 stations is used for identifying of site response and subsequent site classification. Very clear amplification is observed at stiff and soft soil sites. Empirical equations from the literature relating frequency of HVSR peak to average shear wave velocity in the upper 30m(VS30), commonly used as a proxy for site classification, were found to be unreliable, making site classification difficult. To overcome this problem, a soft computing method based on the Generalized Regression Neural Networks (GRNN) algorithm is used. This algorithm relies on HVSR curves at the recording stations and uses pattern recognition against a set of pre-defined curves for different site types. In this approach, site characterization is based on the overall features of the HVSR curves rather than just the peak frequency. The stations are classified into four different site classes, and the results are consistent with local geotechnical information, where available. Rather than relying on VS30, which has been found deficient in many studies, and is known to have difficulty in reliably estimating when geotechnical data is lacking, site classification proposed in this study uses the overall characteristics of HVSR curves of response spectra of recorded strong motion data. This method is proposed as a more reliable and direct method for site characterization in seismically hazardous regions.

A Comparative Study on the VS30 and N30 Based Seismic Site Classification in Kahramanmaras, Turkey

Assessment of seismic site classification (SSC) using either the average shear wave velocity (VS30) or the average SPT‐N values (N30) for upper 30 m in soils is the simplest method to carry out various studies including site response and soil‐structure interactions. Either the VS30‐ or the N30‐based SSC maps designed according to the National Earthquake Hazards Reduction Program (NEHRP) classification system are effectively used to predict possible locations for future seismic events. The main goal of this study is to generate maps using the Geographic Information System (GIS) for the SSC in Kahramanmaras city, influenced by both East Anatolian Fault and Dead Sea Fault Zones, using both VS30 and N30 values. The study also presents a series of GIS maps produced using the shear wave velocity (VS) and SPT‐N values at the depths of 5 m, 10 m, 15 m, 20 m, and 25 m. Furthermore, the study estimates the bed rock level and generates the SSC maps for the average VS values through overburden soils by using the NEHRP system. The VS30 maps categorize the study area mainly under class C and limited number of areas under classes B and D, whereas the N30 maps classify the study area mainly under class D. Both maps indicate that the soil classes in the study area are different to a high extent. Eventually, the GIS maps complied for the purpose of urban development may be utilized effectively by engineers in the field.

Surabaya earthquake hazard soil assessment

The vulnerability of land in an area to earthquake ground motions is one of the factors causing damage caused by the earthquakes. The city of Surabaya, which is crossed by two active fault segments, needs an assessment to reduce the risk of being affected by an earthquake that might occur. The aims of this study are (1) to find out the distribution of Seismic Site Classes, (20 to know the distribution of the value of Seismic Amplification, and (3) to know the potential of liquefaction in the city of Surabaya. Surabaya city, which is geologically dominated by alluvium deposits, consists of soft soil (SE) and medium (SD) sites based on N-SPT30 and Vs30 data. The level of soil amplification against earthquakes ranging from 1 to 4. This occurs because the physical properties of the Surabaya City soil layer are dominated by alluvium deposits. Regions with more than 2 amplification values are located around the coastline on the North and East coasts of Surabaya City. Based on the potential liquefaction index value, Surabaya City is included in the region with a high potential for liquefaction with a potential liquefaction index value of more than 5.

Correlations between Vs and SPT-N by different borehole measurement methods: effect on seismic site classification

The average shear wave velocity of the top 30 m of the subsurface profile (Vs30) is a critical parameter to characterize the seismic site class. Since Vs30 is a quantitative index and measurable by geophysical techniques, the use of Vs30 becomes popular and is widely used in practice. However, the Vs30 of a site may vary due to the different Vs measurement methods used. This could result in a different seismic site class and design force. To quantify the effect of measurement methods on Vs and seismic site classification. This study collected high quality geotechnical investigation reports with standard penetration test (SPT) N-values, and shear wave velocities measured by suspension logging, SCPT, down-hole test and cross-hole logging. The correlations between Vs and SPT-N by different methods were then built and compared. The results show that Vs by suspension logging is relatively high in comparison with those by SCPT and cross-hole logging. The effect on the site classification resulting from the different Vs measurement methods is then demonstrated with several examples.

Comprehensive amplification estimation of the Indo Gangetic Basin deep soil sites in the seismically active area

Site amplification coefficients and acceleration design response spectra (ADRS) for the deep Indo-Gangetic Basin (IGB) have been proposed. Non-linear site response analysis is carried out at 275 deep shear wave velocity (VS) profiles by selecting the input motion based on seismic hazard map for 10% probability of exceedence in 50 years. The spatial variation map of peak ground acceleration (PGA) and spectral acceleration (SA) at surface and various depths for the IGB are also presented. The bedrock PGA of 0.03–0.24 g increased to 0.1–0.97 g at surface and 0.05–0.35 g at a depth of 150 m. Maximum SA is observed between 0.08 s and 0.5 sec. The new site factors viz. Faand Fv are derived for newly defined spectral period range. Further, ADRS is constructed which depends on PGA and seismic site class as per NEHRP. Proposed ADRS is compared with different deep basin studies and BIS:1893.

Prediction of shear wave velocity in underground layers using Particle Swarm Optimization

Shear wave velocity (Vs) is considered a key soil parameter in the field of earthquake engineering. The time-averaged shear wave velocity in the upper 30 m (Vs30) layer of soil is used to classify seismic site class. In-situ Vs test is sometimes unsuitable to the project’s need due to financial reasons, noisy environment on site or simply the lack of expertise. This paper attempts to develop a global prediction model for Vs using Standard Penetration Resistance (Nspt), depth (z) and soil type (st) as the independent parameters. Two approaches to modelling would be taken; a multi-linear regression (MLR) model and an ensemble (EN-PSO) model. The EN-PSO model attempts to improve upon the accuracy of the MLR model prediction ability using the ensemble learning method. A dataset was compiled from literatures for this paper. 5 Base models were developed: MLR, Random Forest (RFR), Support Vector Machine (SVR), Artificial Neural Network (ANN) and k-Nearest Neighbor (KNN) which are combined into an ensemble model named EN-PSO. The weights for EN-SPO was then calculated using Particle Swarm Optimization (PSO). The performance of each models were then compared and it was shown that EN-PSO was the best in terms of: MAE (Mean Absolute Error) = 22.085, MAPE (Mean Absolute Percentage Error) = 9.1 %, RMSE (Root Mean Square Error) = 31.741 and R2 Coefficient of Determination) = 0.895. In addition, it was also shown that the EN-PSO model was able to improve upon the performance of the MLR model, which the most accurate among the Base models. Comparisons were also made between EN-PSO and other suggested Universal Vs correlations and EN-PSO was shown to outperform the other correlation based on prediction using a modified Test set. Three new empirical correlations as alternative for the EN-PSO model was also presented.

Seismic site classification and correlation between VS and SPT-N for deep soil sites in Indo-Gangetic Basin

The Indo-Gangetic Basin (IGB) is occupied by the thick deposits of different types of loose to dense soil due to the active sedimentation. Till date, no detailed study has been carried out on seismic site characterization, and classification considering shear wave velocity >50 m depth. In this study, surface wave survey was used for determining the shear wave velocity (VS) variation with depth at 276 locations in the IGB using low-frequency geophones by performing combined active (Multichannel analysis of surface waves, MASW) and passive (Ambient Noise) surveys. To study the spatial variability of VS, based on the sediment deposition and geological variability, the whole IGB has been divided into Punjab-Haryana region (PHR), Uttar Pradesh region (UPR), and Bihar region (BR). Firstly, using the least square, orthogonal, and mixed effect approaches, a new correlation between the VS and SPT-N has been developed for all the three regions separately. The VS and SPT-N correlation has been developed considering both corrected and uncorrected SPT-N value. The residuals have been determined using the observed and the predicted VS values from newly developed VS and SPT-N correlation from each approach. For the same SPT-N value, correlation for UPR region is predicting high VS value as compared to other two regions. Studying the variation of residuals with VS, it is determined that the mixed effect is performing better for VS ≤ 250 m/s and orthogonal for VS > 250 m/s. At higher N-value, difference between VS values for all the three regions are noticeable. Further, the newly developed relationships are compared with the existing VS and SPT-N relationship. Additionally, the IGB is classified based on the average shear wave velocity in the top 30 m (VS30) as per the National Earthquake Hazards Reduction Program (NEHRP) and compared with Eurocode 8 (EC8). The VS30 is high in the southern part of the BR and UPR and low near to the active channel deposition and major part of the IGB is classified as seismic site class D. Most of the IGB sites have experienced severe damage either due to local site effects or liquefaction during past earthquakes. Therefore, the spatial variability of the average shear wave velocity at different depths, i.e., at top 5,10,15,20, 30 50, 100, 150, 200, 250 and 300 m are also estimated and compared with the available basement depth map.

Seismic site classification and amplification of shallow bedrock sites.

This study attempts to develop empirical correlations between average penetration resistance (), averaged velocities over depth up to bedrock depth () and 30 m () for shallow depth sites (having bedrock at a depth less than 25 m). A total of 63 shallow sites were assessed for penetration resistance values up to the bedrock from Standard Penetration Tests (SPT) and dynamic soil property analysis, i.e., Shear Wave Velocity (VS) from Multichannel Analysis of Surface Waves. The study shows that 30 m averaged shear wave velocities are more than the average velocity up to bedrock depth in shallow bedrock sites because of inclusion of rock site velocity. Furthermore, averaged SPT-N() and average VS () up to bedrock depth were correlated with the 30 m average() values. This is the first attempt in developing empirical relationships of this kind for seismic site classification. These correlations can be made useful for seismic site classification of sites in regions with Standard Penetration Test (NSPT) values and limited VS values. Further surface and bedrock motion recordings of 12 selected KiK-net shallow depth sites were collected and amplifications were estimated with the respective peak ground acceleration, spectral acceleration and thereby related to the average shear wave velocity up to bedrock and 30 m. The results show that the amplification is better correlated to the than for shallow depth sites, and more data can be added to strengthen this correlation.

Seismic site classification

The economic losses left by large recent earthquakes are still considerable, and modern society is wanting not only life protection; it is also demanding that buildings can be immediately occupied after a strong earthquake. The performance-based seismic design allows engineers to design structures with a desired seismic performance for a specified level of hazard. This requires a high standard in the different items involved in the seismic design. One of the key factors is associated with the seismic loads, which are strongly dependent on the local ground conditions. Accordingly, an alternative seismic site classification is proposed, which is based on two dynamic parameters of the ground: the equivalent shear wave velocity, VS30-E, that reproduces the dynamic lateral stiffness of the upper 30 m of the ground, and the predominant period of the site, which is proposed to be estimated applying the H/V spectral ratio of ambient vibration measurements. All the details of this site classification are explained in the paper.